The goal of this trial is to evaluate the safety, feasibility, and efficacy of diltiazem administration in attenuating the natural history of HCM, focusing on tolerability and impact on diastolic function. The primary efficacy endpoint will be an improvement in diastolic function in G+/LVH- subjects receiving active therapy as compared to placebo, as measured by improved mean tissue Doppler echocardiographic early diastolic velocity (E') in the diltiazem group compared to the placebo group 2 years after randomization. As a pilot trial, treatment effects on multiple related parameters including changes in LV dimensions and mass, development of overt LVH, development of cardiac magnetic resonance (CMR) evidence of fibrosis, and levels of serum biomarkers will be analyzed in an exploratory manner to more fully characterize potential treatment effect. The safety endpoint will be no excess of all cause death, cardiovascular death (including sudden death), heart failure requiring medication or hospitalization, or a significant difference in the development of symptoms/side effects which necessitate discontinuation of treatment in the active vs placebo arm.